We have been working on the respiratory system and once more this involved concentration gradients.I more or less understand how it works by now (it's been mentioned a lot in other systems as well) but what intrigues me is that the concentration gradient can be different for several types of molecules in the SAME environment. I don't get that...

For example, in the alveoli, dioxygen molecules get into the capillaries because O2 concentration is lower there. However, CO2 concentration is very high in the capillaries. So how do the O2 molecules "know" that this high concentration within the capillaries is from CO2 and NOT O2? Conversely, how do the CO2 molecules realise that the air in the alveoli is O2-rich but low in CO2...

At first I thought that diffusion depended on 'space', whereby any molecule tries to get as much space around itself as possible and hence moves towards lower concentrations. But the fact that CO2 is present in high concentrations where O2 wants to go is very counterintuitive, because both are molecules that take up space and intuitively should "repel" one another... And instead of that each type of molecule simply 'ignores' the presence of other types.

Thanks for your reply, but there is no mention of any such mechanism at alveoli level, the exchanges there seem to be completely passive. Would you have any references about 'non-passive' exchanges in the alveoli? I have never heard of that and my course never mentions it.

I don't see the problem.It's all about net movement in each direction. If there is more O2 in region A than in region B, there will be more movement of O2 from A to B than from B to A. After all, O2 can't diffuse out of region B if it isn't already present in region B. If B is filled with mostly CO2, then most of the diffusion out of B will be of CO2.

Think about a crowd, with women and men, randomly moving across a sidewalk from one side of a park to the other. If the movement is random, men will gradually diffuse along their gradient, women along theirs, and even though they are all "people," the process still works...

1) the answer of Arlen1991 is completely irrelevant to your question!2) Darby's analogy is perfect3) it's simply about entropy. The molecules move randomly in any direction without any knowledge of presence of other molecules (alike or different). So just by following probability they spread randomly after some time.